Chapter 3: Amino Acids, Peptides, and Proteins: Structure, Properties, and Purification

Amino Acids, Peptides, and Proteins: Structure, Properties, and Purification emphasizes stereochemistry, particularly the L-isomeric forms found in biological systems, and introduces the zwitterionic nature of amino acids at physiological pH. Section 3.2 describes the titration curves of amino acids, allowing for the calculation of pKa values and isoelectric points, which are essential for understanding amino acid behavior in varying pH environments. The concept of the Henderson-Hasselbalch equation is revisited to model acid-base behavior. Section 3.3 focuses on peptide bond formation through condensation reactions and the chemical nature of the peptide bond as a rigid, planar structure due to resonance. The chapter outlines how peptide chains are named and structured from the N-terminus to the C-terminus and introduces simple techniques for protein purification and sequencing, including Edman degradation and mass spectrometry. Section 3.4 details protein classification based on composition and function—globular vs. fibrous, simple vs. conjugated proteins—and explains that primary structure (the sequence of amino acids) dictates higher-level structures. It also discusses the importance of post-translational modifications and functional diversity among proteins. Finally, Section 3.5 introduces the vast range of biological functions proteins perform, from catalysis and structural support to signal transduction and immune defense. Through this chapter, readers gain a foundational understanding of how amino acids combine chemically and structurally to form peptides and functional proteins, setting the stage for more complex discussions of protein folding, function, and enzymology in subsequent chapters.